The present invention relates to a method for the precipitation of nickel as a metallic powder suitable for the production of refined steel from an aqueous solution containing its sulphate. In this method, nickel reduction takes place continuously in one or several autoclaves at a temperature of 80-180xc2x0 C. and hydrogen pressure of 1-20 bar, whereby the production capacity can be raised significantly, compared to batch processes made in correspondingly dimensioned devices or equipment.
The production of nickel from an aqueous solution by hydrogen reduction in autoclaves as batches has been in use on industrial scale since the 1950s. The method is described in articles such as: Benson, B., Colvin, N.: xe2x80x9cPlant Practice in the Production of Nickel by Hydrogen Reductionxe2x80x9d, pp. 735-752 in the conference publication: Wadsworth, M. E., Davies, F. T. (ed.): xe2x80x9cUnit processes in Hydrometallurgyxe2x80x9d, International Symposium in Hydro-metallurgy, Dallas, Feb. 24-28, 1963, Gordon and Breach, New York, 1964. The production method described in the article is still in use through-out the industry and according to the article the method based on the batch principle comprises the following stages: nucleus reduction, reduction and leaching.
In the batch process, nickel nuclei are made in an autoclave by hydrogen reduction using an FeSO4 catalyst. When the nuclei are ready, the mixers are stopped, the nuclei are allowed to settle and the solution on top of the nuclei powder is blown off. In the reduction stage the actual process solution is fed into the autoclave and metallic nickel is reduced from this with hydrogen on top of the nuclei. Reduction typically occurs at temperatures of 199-204xc2x0 C., and at overpressures of 24-31 bar. When reduction has ended, the mixers are stopped, the powder is allowed to settle to the bottom of the autoclave and the solution is removed from on top of the settled powder. The method is repeated 50-60 times and some nickel powder is also removed when the solution is removed. The reduction series or cycle is finished when the particle size of the nickel powder grows so large that its suspension in the autoclave becomes difficult or when the reduction time of one batch becomes too great. At the end of the reduction cycle the whole autoclave is emptied. Any metallic nickel stuck to the inner structure of the autoclave is dissolved off between cycles.
It is clear to a person skilled in the art that the actual reduction stage of the batch process comprises at least the pumping of the pre-heated solution to the autoclave, the hydrogen reduction of the batch of the nickel solution, the settling of the nickel powder and blowing off the residual solution from the top of the nickel powder. All these sub-stages are performed as consecutive actions, not simultaneously. However, only the hydrogen reduction of the nickel solution is effective time from a production point of view and it can be calculated from the above-mentioned article by Benson and Colvin, that this operation uses only 45% of the total time. The capacity of the method can be calculated from this article as:
251 batchesxc3x9746 g Ni/l/(14d* 24h/d)=approx.34 (g Ni/l)/h.
The article by Evans, D. J. I.: xe2x80x9cProduction of Metals by Gaseous Reduction from Solutionxe2x80x9d, Processes and Chemistry, Paper 35/Advances in Extractive Metallurgy, A symposium in London, April, 17-20 , 1967, The Institution of Mining and Metallurgy, mentions that the particle size generated by the nuclei reduction described above is of the order of 0.001 mm.
Metallic nickel production by hydrogen reduction as a continuous process is presented in U.S. Pat. No. 2,753,257. The patent mainly describes reduction in batch processes, but the examples also mention continuous processes. In relation to continuous processes it is stated that a maximum yield of 80% can be achieved and that the batch method should be used for better results. It is characteristic of the said method firstly that the composition of the solution is adjusted twice, and secondly that the iron present in the solution has an adverse effect on the functioning of the method.
In U.S. Pat. No. 2,753,257 the composition of the solution is first adjusted to the optimum demanded for self-nucleation. In the second stage the composition of the solution is adjusted so that it is optimal for the reduction of the metal powder on top of the metal nuclei. It is also supposed in the method that iron is eliminated from the solution by some known method to content levels that do not interfere with the reduction of the metallic powder. The method is performed at a temperature range of 218-232 xc2x0 C. and at 52-55 bar of overpressure.
Another continuous process is presented in U.S. Pat. No. 3,833,351. This describes a method for the production of copper, nickel, cobalt, silver or gold powders from a solution prepared by acid or ammoniacal leaching. Powder production is carried out by reduction with hydrogen gas in a continuous vertical tubular reactor, where the height to diameter ratio of the reactor is at least 10:1. In the patent description it states that powders can be produced in the reactor even in atmospheric conditions. However, the section describing the production of nickel for example reveals that if reduction is carried out in conditions where the average temperature of the reactor is 93xc2x0 C. and the pressure about 32 bar (Table III, Run 2), the resultant solid matter contains only 55% nickel. If economically viable results are required, reduction must be carried out in conditions where the total pressure is for instance in the range of 33 bar and the average temperature 140xc2x0 C. with a maximum temperature of 225xc2x0 C. (Run 1), whereby the amount of nickel powder formed is 90% of the solid matter. The resulting nickel is not only impure but also extremely fine and thus awkward to handle. The size of the powder produced was 0.001-0.002 mm for copper and so small for nickel and cobalt that ordinary settling and filtering may no longer work, requiring perhaps even magnetic separation in order to separate the particles from the solution. The fineness of the powder also greatly hinders washing. This method has never been implemented on industrial scale.
Autoclaves equipped with partitions are used for continuous precipitation and leaching autoclaves, as described e.g. in the article by F. Habashi, Pressure Hydrometallurgy: Key to Better and Nonpolluting Processes, Engineering and Mining Journal, February , 1971, pp. 96-100 and May 1971, pp. 88-94. Partition walls have not been used in reduction autoclaves.
From the above, we can conclude that the nickel hydrogen reduction method has worked relatively well in the batch process, and attempts to convert to a continuous process have been rather poor. The reasons for this have probably been the high temperatures and pressure used in reduction processes, which have made it difficult to change the process over to a continuous one.
A continuous process is cheaper than a batch process, because the production capacity of equipment of the same size is greater than that of a batch process. Now, with the method of the present invention, nickel powder especially suitable as an alloying element for refined steel can be produced by performing continuous hydrogen reduction of a nickel sulphate-containing aqueous solution in a pressurised space in easier conditions than earlier, wherein the hydrogen pressure is in the range of 1-20 bar and the temperature in the range of 80-180xc2x0 C., (preferably with hydrogen pressure from 2-10 bar and the temperature from 110-160xc2x0 C.). According to the invention, at least one autoclave is used as the pressurised space, being equipped with partition walls, which divide it into several sections with mixers, or several consecutive autoclaves with mixers, which autoclaves may be single or multi-sectioned. The invention is particularly advantageous when using nickel sulphate solutions obtained in acid leaching and which therefore do not practically contain ammonium sulphate. The essential features of the invention will be made apparent in the patent claims.
Nickel sulphate-containing aqueous solutions are generally prepared by leaching either nickel concentrate such as laterites or pyrometallurgically produced nickel mattes. The leaching may be either acid or ammoniacal. The nickel content of the sulphate solution usually remains lower in concentrate leaching than in matte leaching, but if liquid-liquid leaching is used as one solution purification step, the nickel content can easily rise to over 100 g/l. In the framework of this invention, reduction is performed from a solution with a nickel content of minimum 30 g/l, preferably at least 50 g/l and most advantageously minimum 80 g/l.
For reduction in a pressurised space, the composition of the nickel sulphate solution feed is adjusted before reduction in a preparation stage, which comprises a number of mixing reactors. The adjustment of the solution composition is carried out only once. If there is any iron in the solution, ferrous sulphate is made use of to form nuclei, on which nickel powder is reduced. If the amount of iron in the solution is not sufficient as it is, iron is added to the solution. In place of iron or in addition to it, chrome can be used for nucleus formation as chrome (II) sulphate CrSO4. Ammonia can also be used for composition adjustment as can the feed of other additives and admixtures normally used in reduction.
If autoclaves divided into sections are used in embodiments of the invention, the upper edges of the partitions are essentially horizontal and their heights from the lowest point of the bottom of the autoclave is graded so that the height of the partition walls seen in the direction of the solution flow decreases, so that the surface of the solution in the sections decreases correspondingly. Gradation can of course be implemented in some other suitable way, for example, so that the partitions are the same height, but have discharge slots or apertures at different heights. The purpose of the partitions is to improve the efficiency of the autoclave.